1. Field of the Invention
The subject invention relates to radiant energy generation and transmission, to radiant energy filtering, to optical filters, to opto-mechanical spatial filters, to fiber optics systems, to methods and apparatus for filtering spatial noise frequencies from a laser beam, and to component mounts.
2. Disclosure Statement
This disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior art inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness, and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material though not necessarily being of itself pertinent. Also, the following comments contain conclusions and observations which have only been drawn or become apparent after conception of the subject invention or which contrast the subject invention or its merits against the background of developments subsequent in time or priority.
For many uses, a laser beam must have a smooth intensity profile. For instance, in optical data processing or transmission, useful information can be completely lost if the input plane illumination is not smooth, while in holography important details can be surpressed or spotty film exposures can produce "dirty" reconstructions. The ideal TEM.sub.oo intensity profile approaches a Gaussian distribution. However, in practice, such "ideal" beam picks up unwanted intensity fluctuations, typically caused by interference effects from light scattered by dust particles in the air, on the mirrors and lenses, and from lens defects. Such intensity fluctuations usually vary rapidly and randomly over distances smaller than beam radius. Focusing of this actual light onto the focal plane of a positive lens forms the optical power spectrum of light distribution in which the higher-frequency noise spectrum is usually separated from the focused Gaussian shape.
It would thus be possible to employ a spatial filter to block unwanted light so as to pass a uniform intensity profile. In this respect, pinholes have been employed for selectively filtering out higher spatial frequencies, while passing a major portion of the total beam power relatively free of noise.
For the theory of operation just provided, as well as for an example of an existing spatial filter, reference may be had to pages 70 and 71 of the 1977-78 Catalog of the assignee of the entire interest hereof. In particular, the assignee's spatial filter shown therein has a pinhole provided in a wafer movable in a spatial coordinate system relative to a focusing lens objective with the aid of three manually adjustable micrometers. In practice, the latter solution represents a rather expensive and complicated approach.
Another existing spatial filter arranged the focusing lens system behind a tiltable mounting plate, as seen from the source of the laser beam. The lens system was disposed in a tubular mount, a rear portion of which extended through the tiltable mounting plate for the reception of the laser beam from the source. A wafer including the pinhole was mounted at a tip of the tubular mount, for tilting jointly with the lens system.
In particular, the mounting plate of the lens system or fixture and of the pinhole was tilted in an endeavor to bring the focus of the laser beam into coincidence with the pinhole by means of a consequent displacement of the pinhole and focus spot relative to each other. In practice, trial and error was employed in seeking an appropriate displacement. However, since in that prior structure the lens system and pinhole were both shifting, albeit by different amounts, the shift of the lens relative to the input laser beam caused the beam to be shifted off axis and the pattern of light through the pinhole to be changed as a result of spatial filter adjustments. In consequence, by the time an operator had found the right place for the pinhole, the beam had shifted laterally and the transmitted light pattern had changed objectionably.
In a different vein, it is known that optical systems have nodal points which are defined as two axial points so located that an oblique ray directed at a first nodal point appears, after passing through the system, to emerge from the second nodal point parallel to its original direction (see HANDBOOK OF OPTICS, by Walter G. Driscoll et al (McGraw-Hill, 1978) p. 2-5. As there stated, the nodal points coincide with the principal points, when an optical system is bounded on both sides by air, as is true in the great majority of applications.
In pinhole-type optical filters, tilting of the lens system and pinhole about the first nodal point could be advantageous, so as to avoid tilting of the focused beam. As the subject invention shows, that principle would, however, not necessarily be transferrable beyond its proper context. Thus, even though both the last-described system and the subject invention employ a kind of panning of the type used in the handling of cameras for varying the location of the image on the film, principles of one system are not readily transferrable to another in the area under consideration.
There is also a need in fiber optics transmission systems of improved methods and apparatus for passing light into fibers in an improved manner, reducing losses and distortions.